1. Field of the Invention
The present invention relates to semiconductor devices and fabrication methods, and, more particularly, to periodically distributed microwave semiconductor device structures.
2. Description of the Related Art.
Semiconductor devices such as transistors are limited by transit time and other effects to frequencies below a few GHz; and thus d-c to microwave power conversion and amplification at microwave frequencies are typically performed by semiconductor devices such as IMPATT diodes and Gunn diodes or tube devices such as magnetrons and gyrotrons. In particular, IMPATT diodes for millimeter wave (30-300 GHz) power generation have played an important role in the advance of millimeter wave systems in fields such as communications, radar, medical research, etc. See, generally, H. Kuno, IMPATT Devices for Generation of Millimeter Waves, in K. Button, Ed., Infrared and Millimeter Waves (Academic Press, 1979), which is hereby incorporated by reference as are all other references cited herein.
However, IMPATT diode junction temperatures should be kept as low as possible for reliable operation; in particular, estimates on silicon IMPATTs indicate an upper limit of 250.degree. C. for safe multiple-year operation. But the small signal admittance (which has negative real part) of an IMPATT generally increases in magnitude as the d-c bias current increases up to current densities in the order of 10.sup.3 to 10.sup.5 A/cm.sup.2. Thus to achieve maximum output power plus reliable operation, the heat dissipated in the diode must be efficiently removed.
Heat removal for an IMPATT is typically accomplished by plating a metal heat sink to the p.sup.+ side of the diode or thermal-compression bonding the p.sup.+ side to a heat sink. But the thermal resistance is inversely proportional to device area, so for higher frequencies (where the area must be minimized to reduce the junction capacitance) the removal of the heat dissipated in the junction limits the power output. Indeed, below about 100 GHz the limitation on IMPATT power appears to be determined by thermal limitations; see Kuno, pp. 95-97.
Further, IMPATT diodes are typically discrete devices mounted in circuits such as reduced height, top-hat resonator, and coaxial waveguides; but such mechanical mounting leads to assembly errors, lack of ruggedness, and impedance mismatches. As the junction thickness is decreased for higher frequency operation, the diode area is decreased to maintain impedance levels: and since the small signal impedance depends upon the reciprocal of the junction capacitance, higher frequency devices are made to have smaller areas. Furthermore, it is more difficult to adjust the parameters of such circuits at high frequencies to produce low impedance levels. See, S. Sze, Physics of Semiconductor Devices p. 607 (Wiley, 2d Ed., 1981) for the frequency scaling of IMPATT parameters. Also, Kuno, p. 97, points out that above about 100 GHz, the limitation on power appears to be determined by adverse effects of diode package and mounting parasitics.
Attempts to overcome such mismatch and mounting parasitics problems with discrete IMPATT diodes include the distributed IMPATT diode in which an extended IMPATT has its p.sup.+ and n.sup.+ layers acting as the plates of a parallel plate transmission line and the depletion layer as the dielectric. See, M. Franz and J. Beyer. The Travelling-Wave IMPATT Mode, 26 IEEE Trans. Microwave Theory and Techniques, 861 (1978) and M. Franz and J. Beyer, The Travelling-Wave IMPATT Mode: Part II--The Effective Wave Impedance and Equivalence Transmission Line, 28 IEEE Trans. Microwave Theory and Techniques, 215-218 (1980). The original distributed diode work appears to be M. Hines, High-Frequency Negative-Resistance Circuit Principles for Esaki Diode Applications, The Bell System Technical Journal 477 (May 1960). However, the large spatial extent of a distributed IMPATT makes the heat dissipated more difficult to remove than that of a small diode for the same current density because of the lack of thermal spreading in the heat sink. Further, the depletion layer of a distributed IMPATT gives an attenuation constant much larger than that of a corresponding typical transmission line because the conductance of the semiconductor is high compared to that of usual transmission line dielectric. Note that the attenuation constant may be approximated at high frequencies as the sum of two terms: one term associated with conductor losses and the other term associated with dielectric losses; and this latter term is of the form 1/2 gZ.sub.0 with g the dielectric conductance per length. See, generally, G. Vendelin, Design of Amplifiers and Oscillators by the S-Parameter Method, p. 65 (Wiley 1982). Indeed, the distributed IMPATT limits device design flexibility because the properties of the diode and the transmission line cannot be separately optimized: for example, the loading of a transmission line is not necessarily uniform for optimum efficiency.
Thus it is a problem to achieve high power efficiency, ease of matching to external circuits, and compact size for microwave diodes.